734 COMBINATION OF GELATIN WITH HYDROCHLORIC ACID 



N = normality of total HCl, a its degree of ionization, and n' = 

 normality of combined HCl. The values which they used for the 

 degree of ionization were conductivity ratios given by Lewis.® 

 Inasmuch as the results of Lloyd and Mayes differed from those 

 obtained by Loeb, it seemed worth while to repeat and amplify his 

 experiments. 



Solutions were made up containing 1, 2.5, and 5 gm. of gelatin in 

 100 cc. of HCl of various concentrations. The gelatin was taken 

 from a stock solution of isoelectric gelatin which had been rendered 

 practically ash-free in the way described by Loeb,'^ and was diluted 

 to twice the concentration required for each set of experiments. 

 25 cc. samples of these solutions were measured out at 33°C. by a 

 pipette, and each sample was diluted to 50 cc. by the addition of the 

 proper amounts of 0.1 M or 1.0 M HCl and water from burettes. 

 The concentration of the gelatin was checked by dry weight deter- 

 minations of the amount of gelatin delivered by the 25 cc. pipette, 

 and was accurate to about 1 part in 200. The concentration of the 

 acid used was determined by titration against pure NaaCOs, and was 

 accurate to 1 part in 500 or better. The pH of the solutions was 

 determined at ?>^° with the hydrogen electrode and potentiometer, 

 using rocking cells of the Clark type connected by a salt bridge of 

 saturated KCl to a saturated KCl calomel cell. The pH values are 

 referred to 0.1000 m HCl as a standard, its pH being taken as 1.036 

 at 33°. The e.m.f. readings obtained were reproducible to within 

 1 millivolt, which corresponds to about 0.02 pH. 



The titration curves obtained in this way are given in Fig. 1, the 

 abscissae being pH values, and the ordinates total concentration 

 of HCl expressed in millimoles per liter, which is the same as cc. of 

 0.1 M acid per 100 cc. The curve for 1 per cent gelatin represents 

 three experiments, one of which was performed by Mr. M. Kunitz. 



In order to determine how much of the total HCl was not combined 

 with the gelatin, a series of solutions containing only HCl and water 

 was prepared, and the pH values were determined. The values are 

 given in Table I. 



^ Lewis, W. C. McC, A system of physical chemistry, London and New York, 

 3rd edition, 1920, ii, 219. 



^Loeb, J., /. Gen. Physiol, 1918-19, i, 39; /. Am. Chem. Soc, 1922, xliv, 213; 

 Proteins and the theory of colloidal behavior, New York and London, 1922, 35. 



